In a spectrophotometer, the wavelength precision and the slit width precision of a monochromator are the most important performance items which determine the characteristics of the instrument. Particularly in high-class instruments, they must be adjusted to theoretical limits in structure and in mechanism. On the other hand, in case where they have been adjusted to the limits in this manner, the adjusting mechanisms need to have especially stout structures so as to endure vibrations during transportation and also fluctuations with the lapse of time.
In any of conventional spectrophotometers, however, wavelength driving means and slit driving means have been operated by mechanical coupling portions, thereby to adjust the wavelength and the slit width.
More specifically, the wavelength is adjusted by a wavelength adjuster which is coupled to a wavelength counter of a wavelength pickup mechanism. The slit width is associated with a slit width indicating panel, and in general, the calibration of the slit width after assemblage has been difficult. Accordingly, there has been the problem that when the mechanism systems shift or have their positions slightly moved during transportation or with the lapse of time, the deviations directly affect the wavelength precision and the slit width, to lower the precision of the spectrophotometer.
As a device for eliminating such problem and for facilitating the calibration of the wavelength of the monochromator, the Applicant has already proposed a monochromator wavelength reading device in Japanese Patent Application No. 50-126906. This device sets a wavelength counter at a fixed count value by the use of a wavelength calibrating pulse signal which appears when the tuned wavelength of the monochromator has become a certain predetermined wavelength .lambda..sub.c, whereby the count value of the wavelength counter is calibrated to the value corresponding to the tuned wavelength of the monochromator.
More concretely, in a wavelength driving device of the sign bar system, a slider which moves on a feed screw is provided with a pawl. When the tuned wavelength .lambda. of the monochromator has become the predetermined wavelength .lambda..sub.c, that is, when the movement distance x of the slider has become x=x.sub.c, the pawel actuates a switch. The actuation signal of the switch is fed to the wavelength counter as the wavelength calibrating pulse signal, to set the count value of the wavelength counter at the fixed value corresponding to .lambda..sub.c. According to the monochromator wavelength reading device, the count value of the wavelength counter is automatically calibrated by the wavelength calibrating pulse signal each time the tuned wavelength of the monochromator becomes .lambda..sub.c to actuate the switch. Therefore, the device has the feature that the count value of the wavelength counter can be automatically calibrated even in case where it has deviated by any cause, for example, that a pulse motor does not operate following up a pulse from a motor driving circuit.
With such calibration based on the wavelength calibrating signal-generator coupled to the feed screw or a gearing, however, there has been the problem that calibrations are impossible for wavelength deviations caused principally by the optical system of the monochromator, for example, by the deviation of an angle defined between a light dispersing element and a lever. As an expedient for wavelength calibrations with such causes taken into account, the Applicant has already proposed in Japanese Patent Application No. 50-126906 a method of calibrating the wavelength by the use of a calibrating light source which emits a known line spectrum. This method needs to additionally dispose the calibrating light source and a sub-detector in an ordinary monochromator, to monitor the line spectrum of the calibrating light source by means of the sub-detector and to separately calibrate the indication of the wavelength counter with an output signal of the sub-detector, and it has had the problem that the optical system of the apparatus becomes complicated. There has also been the problem that, in either of these apparatuses, the slit width calibration cannot be made at all.